Communications networks will generally comprise communication channels, or links, connected by exchanges, or switches. The route taken by data (or voice messages) travelling from one set of terminal equipment via the network to another set of terminal equipment is determined by the switches. The capacity, that is the maximum amount of data, which can be transported in a fixed length of time, can be limited by the mature of the links. However much more significantly, the service which can be provided on those links can be dictated by the exchange design. If a network operator wants to update the services it can offer to customers, it must then first update its exchanges.
This can obviously present a major cost, particularly where a network is extensive. In Britain, looking at the private line area, some 6000 exchanges support one million private wires. Hence the migration of exchanges even to support one million private wires, this being only part of the private wire business provided by British Telecommunications PLC, is both large scale and high profile.
It is not possible to upgrade all of the exchanges at the same time and it is therefore necessary to develop an updating arrangement which will meet a number of constraints. In particular, it is clearly important to minimise upgrade costs but to maximise customer benefit.
This has been a substantial problem in the past. Any exchange upgrading process, apart from the unattainable option of updating 100% of exchanges, will have consequential effects in terminal of the service capabilities available to different terminal equipment. In the past, upgrading arrangements have relied substantially on guess work and have taken of the order of several (for instance 15) hours to generate a solution.